Vascular endothelial growth factor (VEGF) a sub-family of growth factors, more specifically of platelet-derived growth factor family of cystine-knot growth factors. They are important signaling proteins involved in both vasculogenesis (the de novo formation of the embryonic circulatory system) and angiogenesis (the growth of blood vessels from pre-existing vasculature).

Classification

The most important member is VEGF-A. Other members are Placenta growth factor (PlGF), VEGF-B, VEGF-C and VEGF-D. The latter ones were discovered later than VEGF-A, and before their discovery VEGF-A was called just VEGF.

Crystal structure of Vammin, a VEGF-F from a snake venom

A number of VEGF-related proteins have also been discovered encoded by viruses (VEGF-E) and in the venom of some snakes (VEGF-F).

As its name implies, VEGF-A activity has been mostly studied on cells of the vascular endothelium, although it does have effects on a number of other cell types (e.g. stimulation monocyte/macrophage migration, neurons, cancer cells, kidney epithelial cells). In vitro, VEGF-A has been shown to stimulate endothelial cell mitogenesis and cell migration. VEGF-A is also a vasodilator and increases microvascular permeability and was originally referred to as vascular permeability factor.

Alternative classification

Schematic representation of The different isoforms of human VEGF

The broad term 'VEGF' covers a number of proteins from two families, that result from alternate splicing of mRNA from a single, 8 exon, VEGF gene. The two different families are referred to according to their terminal exon (exon 8) splice site - the proximal splice site (denoted VEGF_xxx) or distal splice site (VEGF_xxxb). In addition, alternate splicing of exon 6 and 7 alters their heparin binding affinity, and amino acid number (in humans: VEGF₁₂₁, VEGF₁₂₁b, VEGF₁₄₅, VEGF₁₆₅, VEGF₁₆₅b, VEGF₁₈₉, VEGF₂₀₆; the rodent orthologs of these proteins contain one fewer amino acid). These domains have important functional consequences for the VEGF splice variants as the terminal (exon 8) splice site determines whether the proteins are pro-angiogenic (proximal splice site, expressed during angiogenesis) or anti-angiogenic (distal splice site, expressed in normal tissues). In addition inclusion or exclusion of exons 6 and 7 mediate interactions with heparan sulfate proteoglycans (HSPGs) and neuropilin co-receptors on the cell surface, enhancing their ability to bind and activate the VEGF signaling receptors (VEGFRs).

Types of VEGF and their VEGF receptors.¹

Mechanism

All members of the VEGF family stimulate cellular responses by binding to tyrosine kinase receptors (the VEGFRs) on the cell surface, causing them to dimerize and become activated through transphosphorylation, although to different sites, times and extents. The VEGF receptors have an extracellular portion consisting of 7 immunoglobulin-like domains, a single transmembrane spanning region and an intracellular portion containing a split tyrosine-kinase domain. VEGF-A binds to VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1). VEGFR-2 appears to mediate almost all of the known cellular responses to VEGF. The function of VEGFR-1 is less well defined, although it is thought to modulate VEGFR-2 signaling. Another function of VEGFR-1 may be to act as a dummy/decoy receptor, sequestering VEGF from VEGFR-2 binding (this appears to be particularly important during vasculogenesis in the embryo). VEGF-C and VEGF-D, but not VEGF-A, are ligands for a third receptor (VEGFR-3), which mediates lymphangiogenesis.

Production

VEGF_xxx production can be induced in cells that are not receiving enough oxygen. When a cell is deficient in oxygen, it produces HIF, Hypoxia Inducible Factor, a transcription factor. HIF stimulates the release of VEGF_xxx, among other functions (including modulation of erythropoeisis). Circulating VEGF_xxx then binds to VEGF Receptors on endothelial cells, triggering a Tyrosine Kinase Pathway leading to angiogenesis.

Clinical significance

VEGF_xxx has been implicated with poor prognosis in breast cancer. Numerous studies show a decreased OS and DFS in those tumors overexpressing VEGF. The overexpression of VEGF_xxx may be an early step in the process of metastasis, a step that is involved in the "angiogenic" switch. Although VEGF_xxx has been correlated with poor survival, its exact mechanism of action in the progression of tumors remains unclear.

VEGF_xxx is also released in rheumatoid arthritis in response to TNF-α, increasing endothelial permeability and swelling and also stimulating angiogenesis (formation of capillaries).

VEGF_xxx is also important in diabetic retinopathy (DR). The microcirculatory problems in the retina of people with diabetes can cause retinal ischaemia, which results in the release of VEGF_xxx, and a switch in the balance of pro-angiogenic VEGF_xxx isoforms over the normally expressed VEGF_xxxb isoforms. VEGF_xxx may then cause the creation of new blood vessels in the retina and elsewhere in the eye, heralding changes which may threaten the sight.

VEGF_xxx plays a role in the disease pathology of the wet form age-related macular degeneration (AMD), which is the leading cause of blindness for the elderly of the industrialized world. The vascular pathology of AMD shares certain similarities with diabetic retinopathy, although the cause of disease and the typical source of neovascularization differes between the two diseases.

VEGF-D serum levels are significantly elevated in patients with angiosarcoma (PMID 14746640)

Once released, VEGF_xxx may elicit several responses. It may cause a cell to survive, move, or further differentiate. Hence, VEGF is a potential target for the treatment of cancer. The first anti-VEGF drug, a monoclonal antibody named bevacizumab, was approved in 2004. Approximately 10-15% of patients benefit from bevacizumab therapy, although biomarkers for bevacizumab efficacy are not yet known.

Current studies show that VEGFs are not the only promoters of angiogenesis. In particular FGF2 and HGF [1] are potent angiogenic factors.

Patients suffering from pulmonary emphysema have been found to have decreased levels of VEGF in the pulmonary arteries.

In the kidney increased expression of VEGF_xxx in glomeruli directly causes the glomerular hypertrophy that is associated with proteinuria.²

Anti-VEGF therapies

Anti-VEGF therapies [2] are important in the treatment of certain cancers and in age-related macular degeneration. They can involve monoclonal antibodies such as bevacizumab (Avastin), antibody derivatives such as ranibizumab (Lucentis), or orally-available small molecules that inhibit the tyrosine kinases stimulated by VEGF: sunitinib (Sutent), sorafenib (Nexavar), axitinib, and pazopanib. Both antibody-based compounds are commercialized. The first two orally available compounds are commercialized, as well. The latter two are in clinical trials, the results of which were presented (June 7) at ASCO.

Bergers and Hanahan concluded in 2008 that anti-VEGF drugs can show therapeutic efficacy in mouse models of cancer and in an increasing number of human cancers. But, "the benefits are at best transitory and are followed by a restoration of tumour growth and progression." ³

AZ2171, a multi-targeted tyrosine kinase inhibitor has been shown to have antiedema effects by reducing the permeability and aiding in vascular normalization.

External links

• ResearchVEGF.com
• MeSH Vascular+Endothelial+Growth+Factors
• VEGF antibody

Additionally, VEGF has been show to be associated in the proliferation of inflammatory cytokines in some strains of atopic dermatitis

References

Further reading